In the can making industry, different approaches exist for transferring cans between processing turrets in continuous motion machinery. One approach employs a vacuum transfer starwheel, an example of which is described in U.S. Pat. No. 7,418,852, titled “Quick Change over Apparatus for Machine Line” and issued on Sep. 2, 2008, the contents of which are incorporated entirely herein by reference.
The vacuum transfer starwheel receives a can from a first processing turret into a recess and rotates to move the can to a position where it can be transferred to another processing turret. Vacuum is used to secure the can in the recess against gravity and/or centrifugal force as the starwheel rotates. The recess is typically designed with a diametric clearance that makes it easier for the can to enter and exit the recess as it is transferred between processing turrets.
Conventional transfer starwheels can generally handle a range of can lengths as long as the center of gravity of the can remains inside the recess. Depending on the rotational speed of the transfer, the weight of the can, and the vacuum supply, the center of gravity may also reside outside the conventional location provided that the article remains under control during rotation. Thus, when one wants to use a particular can processing machine to handle a can with a different length, the transfer starwheel may need to be repositioned in the machine or may need to be completely replaced with another transfer starwheel to ensure that the center of gravity of the new can remains in the recess or under control. As described in U.S. Pat. No. 7,418,852, a can processing machine may be reconfigured with the addition/replacement of a starwheel segment on quick-change machinery.
When transferring a can between processing turrets, the position where the can is received into the recess (pick-up position) and the position where the can is released (drop-off position) are important, especially as the speed of the machine increases. At high speeds, however, the can may have a tendency to migrate axially in position due to windage, external forces, a slick interface between the can and the starwheel, etc. A transfer starwheel can minimize such migration by applying a sufficient vacuum to the can.
To minimize vacuum leakage between the can and the transfer starwheel and to apply a sufficient vacuum to the can, the can must be properly seated in the recess of the transfer starwheel. In other words, the transfer starwheel must receive the can into the recess with reasonable accuracy to apply the vacuum and maintain control across the full speed range of the machine. An accurate fit between the can and the recess is more easily achieved when the can is substantially cylindrical. Substantially cylindrical cans, for example, include cans where the outside diameter of the sidewall of a middle section is substantially uniform except for the uppermost section and lowermost section of the can. The substantially cylindrical shape of the starwheel recess and the internal vacuum cavity geometry accommodate the substantially cylindrical can for effective transfer between processing turrets. In general, conventional starwheel designs are better suited to handle substantially cylindrical cans.
On the other hand, conventional transfer starwheels may be less effective when handling “shaped” cans. A “shaped can” or “shaped container,” as used herein, refers to a can or container whose sidewall at its middle section does not have a substantially uniform diameter (non-cylindrical). When the non-uniform sidewall of the shaped can is received into the starwheel recess, it is more difficult to position the shaped can accurately within the recess and to minimize vacuum leakage. To address this problem, the starwheel recess may be machined to match the can profile more exactly. Such a solution, however, is far from cost effective. In particular, if any aspect of the can geometry is changed, a replacement starwheel must be machined to accommodate the new geometry. Moreover, even if a profiled recess is employed, seating problems may still occur if the can migrates axially in position or if the shape becomes non-asymmetric due to process variabilities. Such seating problems may require the application of a greater vacuum which undesirably increases operating costs. In addition, the profiled recess does not completely address the inherent instability of the shaped can over the full range of machine speeds.
In general, the process of making a can must provide a degree of latitude in product form, as cans vary in material structure, grain direction and material flow. Furthermore, the process of making a can must be sufficiently flexible to accommodate changes in shape for branding or other purposes without incurring unacceptable costs and requiring significant time and effort.